Tolerance-based transplantation protocols on costimulation blockade have achieved remarkable success in reversbased ing chemical diabetes in rodents. Our laboratory s protocol combines a donor-specific transfusion (DST) with anti-CD154 mAb. It is not clear, however, that costimulation-blockade-based protocols will be successful when applied to the cure of autoimmune type 1 diabetes because they have all failed in the NOD mouse. To address this critical problem, we have developed a new a model system to separate the mechanisms that control autoimmunity from those that control peripheral tolerance induction. This system is based on the key observation that the resistance of NOD mice to allograft tolerance induction, in contrast to the expression of autoimmunity, is a dominant trait in (NOD x C57BL/6)F1 animals. These mice are autoimmunity-free but express NOD defects in dendritic cell maturation and in the response of alloreacfive CD4 + and CD8 + T cells to tolerance induction. Taking these observations as a point of departure, this project will test the hypothesis that the mechanisms that control the induction of transplantation tolerance by co-stimulation blockade differ from those that control the loss of self-tolerance and expression of autoimmunity. Specific Aim 1 is to identify the cellular basis for defects in tolerance induction in (NOD x C57BL/6)F1 mice. It will test the hypothesis that the dominant resistance of(NOD x C57BL/6)F1 mice to allografi tolerance induction is due to defects in dendritic cell maturation. Aims 2 and 3 will use new alloreactive "synchimeric" model systems. "Synchimeras" circulate trace levels of"marked" alloreactive transgenic T cells in syngeneic but genetically unaltered mice that express the genetically dominant transplantation tolerance resistance trait of NOD mice in the absence of autoimmunity. Specific Aim 2 is to investigate the mechanisms by which tracer KB5 alloreactive CD8 + T cells in synchimeric (NOD x KB5 CBA)F1 mice resist tolerance induction. We hypothesize that the relative resistance of alloreactive CD8 + T cells to deletion by costimulation blockade is a major barrier to tolerance induction. Specific Aim 3 is to investigate the mechanisms by which tracer D10 alloreacfive CD4 + T cells in (NOD x D10 B10.BR)F1 mice resist tolerance induction. We hypothesize that a second barrier to the induction of tolerance in (NOD x C57BL/6)F1 mice is a defect in the response of alloreactive CD4 + T cells to co-stimulation blockade. The knowledge generated by these studies should foster development of co-stimulation blockade-based tolerance induction protocols applicable to the treatment of all forms of human type 1 diabetes by islet transplantation.